1. Field of the Invention
This invention relates to a method and apparatus for preserving or restoring audio to video synchronization, or lip sync, in television systems which use video processing devices such as frame synchronizers that generate large time delays in the video signals which are processed.
2. Description of the Prior Art
With the advance of television technology, and in particular the development of complex systems for processing the video portions of television signals, time delays of video signals which are passed through these processing systems are increasing to a point where a noticeable delay in said video signals with respect to their associated audio signals, (lip sync) can be generated. One typical situation where this problem may arise is in the transmission of network programming. Normally, network programming in some parts of the U.S. is relayed by microwave links from one station to the next in a serial fashion. It is not uncommon for the network program to pass through a string of ten or more stations. With the development of relatively low cost video frame synchronizers, it is possible that each of the stations in the string will pass the video portion of the network program through a synchronizer. Each synchronizer in the string will generate a delay in the video signal which can continuously vary from a minimum of several microseconds to a maximum of approximately 1/30 second. If ten synchronizers in the string were all at maximum delay a video delay of approximately 1/3 second is generated, which also causes a 1/3 second lip sync problem.
Normally, in electronic systems, the changes of delay times of associated signals can be compensated for by inserting fixed or manually adjustable delay devices in one of the signal paths, and there are several commercially available devices for delaying audio signals by fixed amounts. In the particular application of television systems, fixed or manually adjustable delays are unsuitable for insertion into the audio channel to restore preper video to audio synchronization, or lip sync, because in many instances the video delay through a particular processing device is constantly changing.